Vehicles having an internal combustion engine can operate in a variety of modes. As one example, an engine may operate in a spark ignition (SI) mode, wherein a charge of a mixture of air and fuel is ignited by a spark performed by a sparking device within a combustion chamber. As another example, an engine may operate in a compression ignition mode, wherein a mixture of air and fuel are compressed within a combustion chamber by a piston, causing ignition of the charge without necessarily requiring the addition of a spark from a sparking device.
One type of compression ignition known as homogeneous charge compression ignition (HCCI) utilizes compression of a substantially homogeneous mixture of air and fuel to achieve controlled auto-ignition (CAI). In HCCI engines, ignition occurs virtually simultaneously throughout a combustion chamber as a result of compression instead of spark ignition, making the combustion process challenging to control. HCCI engines are similar to conventional gasoline engines in having a homogeneous charge, but are similar to conventional diesel engines in having compression ignition. HCCI engines may be used to combine gasoline engine low emissions with diesel engine efficiency.
HCCI combustion engines typically change operation conditions more slowly than other combustion processes. The engine hardware used to control initial cylinder conditions such as internal residuals, intake air temperatures, and the combustion process stability window, limits dynamic response. Further, during HCCI operation and while transitioning to and from an HCCI combustion mode, undesired torque pulses can negatively impact driving torque response and noise, vibration and harshness.
In one approach, as described in U.S. Pat. No. 6,390,054, issued to Yang, an engine controller smoothes transitions between combustion modes by transitioning a first cylinder group between combustion modes and at a different time transitioning a second cylinder group between combustion modes. In another approach, as described in European Patent Application EP 1612393, by Almkvist et al., a method is provided for controlling a four-stroke multi-cylinder spark ignition combustion engine using cylinder deactivation of a subgroup of cylinders while another subgroup is working, during operation with an engine torque below a predetermined level. Almkvist further discusses a low engine speed strategy of operating inlet and exhaust valves normally and gradually opening a throttle until maximum engine stability is achieved.
However, the inventors herein have recognized disadvantages with either approach. First, an approach that smoothes undesired torque pulses while transitioning two separate cylinder groups between an HCCI combustion mode and a non-HCCI combustion mode, does not address undesired torque pulses arising from non-HCCI cylinders when an engine is no longer transitioning cylinders between combustion modes. Second, an approach that operates engine valves normally for non-HCCI cylinders and modulates a throttle for torque balancing affects all cylinders served by the throttle, irrespective of if the cylinders operate in HCCI mode.
The inventors herein have recognized the above-mentioned disadvantages and have developed a system that improves torque delivery in an engine with a first portion of cylinders operating in HCCI combustion mode and a second portion of cylinders not firing.